Concrete Ringwall Design for Water Storage Tank

[tocaalbe]

Hello,

I am designing a concrete ringwall foundation for a water storage tank. I am not sure how to check the ringwall for the seismic forces. I am using AWWA D100-96. The effective mass of the water and the heights at which the resulting design horizontal forces are assumed to act may be calculated using Sec. 13.3.3.2. When I read Sec. 13.3.3.2 it provides a method using curves to solve for the heights from the bottom of the tank shell to the centroids of the lateral seismic forces. Sec. 13.3.3.2.2 indicates the moment arms computed are valid for tanks supported on ringwalls. These forces and height are computed from the curves using the ratio D/H. I am able to solve for the base shear and overturning moment due to the seismic forces applied to the bottom of the tank shell. However, a note in this reference indicates that the tank foundation is subjected to an additional overturning moment due to lateral displacement of the tank contents. Has anyone used AWWA D100-96 that can provide some help?

 

[BigH]

Why are you using a concrete ringwall? Can not the tank sit on a pad of engineered granular fill extending 5 to 10 ft beyond the edge of the tank?

 

[tocaalbe]

The firm I work for always uses a ringwall for water storage tank foundations. I have many examples of ringwall designs here other engineer's have done but none include the seismic requirements.

 

[BigH]

tocaalbe - Okay, but why does your firm always use a ringwall? Force of habit? ?? This is a debate I've been having with others over the years - I've never been involved with an oil or bulk storage tank, clarifier, etc. with ringwalls. Just curious as to the reasoning.

 

[tocaalbe]

BigH, I really wish I had a definitive answer for you. Perhaps it is force of habit. That's something that you have never used a ringwall. I have files of calculations going back about 28 years here and all of them used a ringwall. Most all of the tank calculations I have seen require anchorage. Perhaps that's a way of anchoring the tank?

 

[bimr]

BigH, the manufacturer's of the bolted tanks use ringwalls in thier designs. This would consist of an 8" thick concrete floor and a ring wall foundation to frost depth. There is an embed in the ringwall to attach the wall plates onto.

 

[givir]

Did you check Api 650? I think there is something about seismic force in Appendix E

 

[BigH]

bimr: Perhaps for small tanks? I would think that a concrete slab under, say, a 150 ft diameter tank sitting on firm clayey soil would end up cracking badly. We sat our large dia steel tanks on a 5 ft pad of granular engineered fill - the pad extended out some 10 ft beyond the edge of the tank - this puts frost protection below the tank by soil thickness. Still, I do believe that it seems to becoming a force of habit - standard designs. With no ring wall, you wouldn't have to structurally design a ringwall to a seismic condition. Just make sure that the soil conditions below the tank on granular pad doesn't liquefy. Chinese I knew had standard bridge designs. If the site conditions didn't match the bridge requirement, they would change the site conditions to suit.